Severe acute respiratory syndrome (SARS) has recently emerged as a new human disease with a mortality rate as high as 15%. The genome of the putative etiological agent, SARS-associated coronavirus (SCV), contains a spike glycoprotein that is implicated in viral attachment and entry by homology to similar proteins in related coronaviruses. These spike glycoproteins contain two regions of hydrophobic heptad repeat sequences characteristic of the viral trimer-of-hairpins structural motif. Formation of a trimer-of-hairpins is an essential step in the membrane fusion process of other enveloped viruses, including HIV-1, Ebola virus and respiratory syncytial virus. Moreover, for these unrelated viruses, polypeptides and small molecules that disrupt the formation of this crucial structure are potent inhibitors of viral infectivity. The studies proposed here test the trimer-of-hairpins structural hypothesis for SCV and develop membrane fusion assays to test possible inhibitors of SCV spike glycoprotein. The specific aims are: 1) To generate and biophysically characterize polypeptides derived from the heptad repeat regions of SCV spike glycoprotein, and to solve the structure of particularly stable constructs using x-ray crystallography; and 2) To develop a cell-cell fusion assay and/or a pseudoviral infectivity assay to assess the fusion activity of the SCV spike glycoprotein, and to test polypeptides derived from the heptad repeat regions for potential inhibitory activity. These experiments will explore the structure and function of the SCV spike glycoprotein, essential information for the future development of antiviral therapeutics and vaccines.